In recent years, along with a rising level of integration in semiconductor devices, circuit patterns of transistors and so on configuring these semiconductor devices are being increasingly miniaturized. Required in this miniaturization of the patterns is not simply a thinning of line width but also an improvement in dimensional accuracy and positioning accuracy of the patterns. This trend applies also to semiconductor memory devices.
Conventionally known and marketed semiconductor memory devices such as DRAM, SRAM, and flash memory each use a MOSFET as a memory cell. These semiconductor memory devices require, along with miniaturization of the patterns, an improvement in dimensional accuracy and positioning accuracy at a rate that exceeds a rate of the miniaturization. As a result, a large burden is placed also on the lithography technology for forming these patterns which is a factor contributing to a rise in product cost.
Furthermore, resistance variable memory is attracting attention as a candidate to succeed such semiconductor memory devices employing a MOSFET as a memory cell. The resistance variable memory herein includes not only ReRAM (Resistive RAM) but also phase change memory (PCRAM: Phase Change RAM). The ReRAM uses a transition metal oxide as a recording layer to store a resistance state of the transition metal oxide in a nonvolatile manner. The phase change memory (PCRAM) uses chalcogenide or the like as a recording layer to utilize resistance information of a crystalline state (conductor) and an amorphous state (insulator).
Two kinds of variable resistance elements in the aforementioned resistance variable memory are known, namely a unipolar type and a bipolar type. In a bipolar type resistance variable memory, the variable resistance element is applied with a voltage pulse (write pulse, erase pulse) of different polarity for a setting operation (write) and a resetting operation (erase). On the other hand, in a unipolar type resistance variable memory, the polarity of the voltage pulse applied in the setting operation and the resetting operation is the same, and what differs between the setting operation and the resetting operation is an amplitude and time of the applied voltage pulse.
In a conventional resistance variable memory, there is a problem that during application of the erase pulse to the variable resistance element, write is mistakenly re-performed after erase has been performed (so-called incorrect write). This problem has not been sufficiently solved. In particular, in a unipolar type resistance variable memory, a difference in amplitude and time of the applied voltage between the setting operation and the resetting operation is small, and there is therefore a large risk of an incorrect write occurring after completion of the resetting operation. Even in bipolar type resistance variable memory, the risk of incorrect write has not been sufficiently reduced. Hence, there is a need for proposal of a resistance variable memory having a small risk of incorrect write.